This invention relates to time domain continuous transmit active sonar, and, more particularly, to such sonar wherein the acoustic energy generated by the sonar for coupling into the transmitting medium (typically water) includes predetermined frequency interruptions or discontinuities in the frequency domain, and further wherein processing techniques may be employed for permitting reduction of the amount of acoustic energy required for obtaining equivalent performance with prior active sonar systems.
A basic limitation of a typical active pulsed, or pinging, sonar is that transmission and reception cannot occur simultaneously. For applications where simultaneous transmission and reception are desired, a continuous transmission frequency-modulated sonar such as described in "The Detection and Classification of Objects Lying on the Seafloor"--deRoos et al, J. Acoust. Soc. Am. 84(4) (1988) may be used. However, such prior continuous wave systems experience an inherent range Doppler ambiguity phenomenon when only the transmit/receive time and frequency of the return echo are considered.
Further, in the anti-submarine warfare (ASW) arena, arrays of passive acoustic listening devices, such as hydrophones, are often towed by surface ASW ships. The arrays capture acoustic energy incident thereon, convert the acoustic energy to representative electrical signals and supply the resulting electrical signals to the associated ship for processing, such as target detection and tracking. A desired design goal and operational feature of some seagoing vessels including submarines is to achieve totally silent operation, at least while stationary or moving under their own power at a relatively slow speed, say less than about 5 knots. As the operational noise from a potential target vessel is reduced, the effectiveness of the towed arrays in detecting potential targets is decreased.
Inasmuch as towed array passive acoustical systems are widely available, it would be desirable to use the existing arrays and to increase the effectiveness thereof, especially with respect to quieter potential targets, without having to invest in a design of an entire sonar system.
Accordingly, it is an object of the present invention to provide a continuous transmit frequency modulated sonar system, wherein range ambiguities may be readily avoided and/or properly resolved.
Another object of the present invention is to modify or retrofit an existing towed array passive sonar system for increasing its target detection capabilities and providing accurate range determination.
Still another object of the present invention is to provide a continuous transmit frequency modulated sonar system wherein the frequency of the transmitted wave of energy may be rapidly changed while maintaining target detection and accurate range determination.
Yet another object of the present invention is to include a frequency interrupt continuous transmit active sonar in combination with a towed array passive sonar system for increasing target detection capabilities.